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An international journal of news from the stellarator community
Editor: James A. Rome Issue 161 April 2018
E-Mail: James.Rome@stelnews.info Phone: +1 (865) 482-5643
On the Web at https://stelnews.info
Wendelstein 7-X status update
First divertor operation: higher plasma densities,
longer discharges
Shortly before Christmas, the 2017 experimental campaign
on Wendelstein 7-X (W7-X) was completed as
planned. Starting in early September, fifteen weeks of
operation were conducted and major machine components
worked without failures, allowing the scientists an efficient
use of the machine time. The W7-X team currently
comprises about 150 scientific staff members based in
Greifswald and more than 60 scientists from EUROfusion
laboratories, the United States, Japan, and Australia.
Expectations for the experiments were high: after the first
campaign, ending in March 2016, W7-X was equipped
with a new component for plasma exhaust – the so-called
island divertor. The divertor takes up heat loads conducted
along magnetic field lines onto its special targets that
interact with magnetic islands external to the confinement
volume in which nested magnetic surfaces exist (see Fig.
1).
The test divertor was installed for the recently conducted
campaign and experiments to be carried out in 2018, and is
part of a long-term strategy to extend plasma pulse length.
Prior to this experimental campaign, ten divertor units had
been installed. Even without water cooling, the test divertor
units are more tolerant to unexpected loads and thus an
ideal tool for first divertor tests. Consequently, the goals of
this divertor phase were to gain experience with the magnetic
field in the presence of the divertor and to develop
safe and reliable operation. This will form the basis for
experiments that will ultimately extend pulse lengths from
seconds to several minutes.
Figure 2 shows how the plasma acts on the divertor. The
figure shows thermography data indicating the location of
the heat loads. During the campaign the maximum surface
temperature on the divertor approached 900 °C after a few
seconds of the discharge. A first analysis indicates that the
observed temperatures match theoretical predictions. Having
gained confidence that the heat loads can be controlled,
longer discharges of up to 30 s became routine by
the end of the campaign. With the longer discharges, the
divertor allowed deposit of up to 75 MJ of heating energy
in W7-X – more than 18 times the energy limit of the first
campaign.
Fig. 1. Wendelstein 7-X plasma (yellow) with divertor
(green); plasma cross-section with magnetic islands cut by
divertor targets (green).
In this issue . . .
Wendelstein 7-X First divertor operation: higher
plasma densities, longer discharges
With the new (as yet uncooled) divertor installed, discharges
up to 30 s became routine, and 75 MJ of
energy could be absorbed by the ten divertors. The
magnetic field was tweaked to equalize the power
load on all the divertors. Pellet injection was used to
raise core plasma densities to 1.4  1020 m3. ........ 1
2017 International Stellarator–Heliotron Workshop
The 2017 International Stellarator–Heliotron Workshop
(ISHW-2017), hosted by the Heliotron J group
[Institute of Advanced Energy at Kyoto University],
was held at Kyoto University 2–6 October 2017.
Stellarator News -2- April 2018
However, in order to successively extend the heating
power and pulse lengths, small deviations from the ideal
magnetic fields leading to asymmetric power loads needed
to be overcome. These deviations are due to small inaccuracies
in the construction of the superconducting coils of
Wendelstein 7-X, but small magnetic field errors can be
corrected with a set of supplementary trim coils. Using
these coils, we successfully equalized the heat load to each
divertor unit as confirmed by the infrared cameras.
With these symmetrized power loads it was possible to
extend the heating energy. With correspondingly extended
heating power, the plasma density could also be increased.
Core plasma densities up to 1.4  1020 m3 were achieved
— more than four times as much as in the previous campaign.
Two developments made this achievement possible:
1. A new system for efficient fueling of Wendelstein 7-X
plasmas (the pellet injection system) was successfully
brought into operation. The injector shoots tiny frozen
hydrogen pellets into the plasma, as illustrated in Fig. 3.
As the pellets propagate through the plasma, they are
ablated and finally ionized, thereby fueling the plasma.
2. A new heating scheme is required at high densities. A
special polarization of the microwave beams allows them
to penetrate into the plasma center beyond regions of high
plasma density, where the beam would otherwise be
reflected. The microwave beam polarization is changed
during the initial 2 s of plasma operation, when densities
are lower. This heating scheme creates plasmas with
excellent energy confinement and high ion temperatures.
Finally, the capability of plasma operation at high plasma
density appears to be a key ingredient to operate the divertor
under favorable conditions — the so-called detachment
regime. This is the state of the divertor plasma in which
the divertor target plates are well isolated, preventing
higher heat fluxes from reaching the target plates. Divertor
detachment may be a requirement for future fusion power
plants, because it significantly minimizes the power loads
to the divertor surface. In this campaign, we could reach
stable, complete detachment for several seconds. This
achievement results in a reduction of the power loads by a
factor of 10 on all ten divertors.
Additional physics aspects also benefited from the extension
of achievable plasma densities at different heating
power. Crucial aspects of stellarator optimization such as
the control of internal plasma currents and the stellarator
specific heat transport could be addressed. New and exciting
insights into plasma turbulence and the flows of impurities
were possible with new and upgraded diagnostics
systems. All experiments performed included variations of
the magnetic field configuration, which is an important
parameter influencing the plasma transport and stability
properties.
In summary, the 2017 experimental campaign has been
successfully completed. The time until summer 2018 will
be used for completing and commissioning new installations
including new plasma diagnostics, a specially
designed divertor element (the so-called scraper element),
Fig. 2. Infrared image of one of ten divertor units during a
plasma discharge in W7-X. Two bright stripes indicate the
most highly loaded areas, called strike lines. These strike
lines are typically a few centimeters wide and reach a temperature
of up to more than 400 °C in this example.
Fig. 3. View into the plasma vessel of W7-X during pellet
injection. Tiles of the structured wall protection are seen in
the lower central region illuminated by the visible plasma
light (blurry regions). The pellet is indicated by the arrow.
Stellarator News -3- April 2018
and new heating systems.The present plan foresees that
plasma operation will be commenced in July 2018.
Dr. Andreas Dinklage for the W7-X Team
Max-Planck-Institut für Plasmaphysik Greifswald
Wendelsteinstraße 1
17491 Greifswald , GERMANY
E-mail: w7xnewsletter@ipp.mpg.de
2017 International Stellarator–
Heliotron Workshop
The 2017 International Stellarator–Heliotron Workshop
(ISHW-2017), hosted by the Heliotron J group [Institute of
Advanced Energy at Kyoto University], was held at Kyoto
University from October 2 to October 6, 2017. The conference
was locally organized by a team from Kyoto University
coordinated by Prof. Mizuuchi (chair) and Prof.
Nagasaki (secretary). The conference attracted nearly 200
delegates from the stellarator-heliotron community, as
well as invited speakers from the tokamak community.
Our Japanese hosts provided an excellent and unique
atmosphere for the conference.
The international scientific program committee, chaired
by C. Hidalgo, played a key role in the development of the
scientific program [http://www.center.iae.kyoto-u.ac.jp/
ishw2017/program.html]. The objective of the conference
was to assemble contributions with depth and high quality
from the entire stellarator-heliotron community, including
a special session on the physics of decoupling transport
channels to promote synergies between tokamaks and stellarators.
There were four important highlights since the previous
workshop held in Europe [Greifswald, October 2015]:
• The first experimental campaign on W7-X has been conducted
(with an uncooled limiter in the OP1.1 campaign)
with a program dedicated to the investigation of neoclassical
physics and the effect of correction coils on the limiter
loads [Lazerson et al.] followed by (during fall 2017 and
on-going when the conference was held) operation using
an inertially cooled graphite divertor (OP1.2a campaign).
The successful W7-X start confirmed that a complex magnetic
topology can be achieved with an accuracy better
than 1:100,000 [Otte et al.]. Among the achievements
were the first demonstration in W7-X of a reduced bootstrap
current in optimized magnetic field configurations,
and global confinement that matched well with the ISS04
scaling. These findings provide the first experimental evidence
that the basic elements of stellarator optimization
work as predicted [Sunn Pedersen et al.; Dinklage et al.].
• LHD isotope effect studies have successfully started in
2017 after several years of preparation [Morisaki et al.,
ISHW-2017]. There are some indications of improvements
in electron confinement [Tanaka et al.], whereas gyrokinetic
analysis predicts improved confinement due to zonal
flow stabilization of trapped electron modes (TEM) in
deuterium plasma [Nakata et al.]. The ion temperature
could be extended, reaching 10 keV. The LHD isotope
effect program has boosted a dedicated program on isoStellarator
News -4- April 2018
tope physics in mid-size devices such as Heliotron-J (H-J)
[Ohshima et al.] and TJ-II [Losada et al.].
• A potential threat to the performance of stellarators and
heliotrons is the problem of impurity accumulation.
Results presented during the conference have shown that
temperature screening of impurities and moderation of
neoclassical impurity accumulation are possible in relevant
operating regimes and that flux-surface variations of
electrostatic potential can have a significant impact on
high-Z impurity radial fluxes [Mollén et al.; García-
Regaña et al.; Velasco et al.].
• With the advent of neoclassically optimized stellarators,
optimizing them for turbulent transport is a key next step.
Mechanisms to reduce turbulent transport [Hegna et al.]
and validation of gyrokinetic calculations [Anderson et al.;
Sánchez et al.] were presented during the workshop.
The conference offered a keynote talk by O. Motojima on
“Development of Fusion Energy Research in the Context
of Stellarator and Heliotron” and two poster sessions and
contributed papers covering a wide range of topics in the
fields of core transport [Morisaki et al., Dinklage et al.],
plasma-wall and limiter/divertor physics [Bader et al;
Motojima et al.; Masuzaki et al.; Wenzel et al.; Winter et
al.], impurity [Kawamura et al.; Nunami et al.] and particle
transport [Panadero et al.], fast particle dynamics [Ogawa
et al.; Yamamoto et al.], operational limits [Volpe et al.],
magnetic topology effects [Mizuuchi et al.; Michael et al.],
coil design and error field prediction [Zhu et al.], 3-D field
effects [Prebeton et al.] and equilibrium reconstruction
[Cianciosa et al.], plasma transitions [Dreval et al.; Takahashi
et al.], heating scenarios [Grekov et al.; Wolf et al.],
neoclassical modeling of flows and electric fields [Kumar
et al.], and integration [Goto et al.]. Contributed papers
and posters covered both the engineering approach to control
and improve performance of magnetic confinement, as
well as the physics approach, oriented at providing basic
understanding in order to predict the behavior of burning
plasmas with confidence.
The special scientific session on transport decoupling
mechanisms included contributions from stellarators [Ida,
Helander] and tokamaks [Hubbard, Kamada], followed by
a general discussion led by Hidalgo. There are some conflicts
in the optimization criteria for fuel, energy, and
impurity control since the desire is fuel peaking and an
increase in energy confinement, together with lack of
accumulation and peaking of impurities (including He
with a strong core source). Then, validation of mechanisms
that might decouple energy/particle/impurity transport
channels is an important open question in
which synergies between tokamaks and stellarators should
be explored. The present state of theoretical research on
transport decoupling mechanisms was presented by Helander.
The phenomenology and physics of I-mode transport
decoupling in tokamaks were reported by Hubbard.
Kamada presented an overview of research regimes and
status of JT-60SA. Impurity transport in various stellarator/
heliotron devices was reviewed by Ida from the viewpoint
of ion particle transport decoupled from the electron
transport. Strategies to study the importance of kinetic
(e.g., quantifying particle and energy transport driven by
fluctuations at different energies) and fluid (e.g., phase
relation between density and temperature fluctuations)
effects were addressed during the discussion session.
The local organizing committee gave two awards to (not
invited) PhD students: to Dorothea Gradic (Max Planck
Institute for Plasma Physics, Greifswald, Germany) for her
contribution on “Doppler Coherence Imaging of Divertor
and SOL flows in W7-X and ASDEX-Upgrade” and to
Caoxiang Zhu (University of Science and Technology of
China) for “Hessian Matrix Used for Stellarator Coil
Design and Error Fields Prediction.”
C. Hidalgo,1 D. Gates,2 R. König,3 S. Kubo,4 C. Michael,5
T. Mizuuchi,6 K. Nagasaki,6 V. Pustovitov,7and V. Voitsenya8
1CIEMAT, Spain
2PPPL, USA
3IPP, Germany
4NIFS, Japan
5ANU, Australia
6Kyoto University, Japan
7NRCKI, Russia
8Kharkov, Ukraine.